Place the watermelon cubes into a mixing bowl and mash them with a fork or potato masher until you have a roughly uniform pulp with some texture remaining.
Add the granulated sugar and lemon juice to the watermelon pulp, then stir thoroughly until the sugar dissolves completely and the mixture is well combined, about 1-2 minutes.
Pour the mixture onto a flat baking sheet and spread it evenly with a spatula to about 1-inch thickness, ensuring a uniform layer for quick freezing.
Place the baking sheet in the freezer and freeze for about 2 hours, or until the edges are icy and crystals begin to form, checking periodically to prevent overfreezing.
Use a fork to scrape the frozen mixture into icy flakes, starting from the edges and working inward, creating a crumbly, crystalline texture with crunchy edges and melty insides.
Serve immediately in bowls, garnished with a small mint leaf if desired, for a refreshing icy treat.
